Erythropoiesis will be characterized during ontogeny, and in patients with hematologic disorders. The hypothesis is that erythrocytes produced in fetuses differ from the fetal-like erythrocytes produced during stress erythropoiesis, in bone marrow failure or in hemoglobinopathies. One approach to treatment of such patients would be to elicit selective growth of fetal-like progenitors, with maturation into fetal-like erythrocytes. This requires demonstration that there are specific classes of erythrocytes and erythroid progenitors, and the specific types can undergo selective expansion. Erythrocytes will be examined for fetal, fetal-like, and adult properties ( levels of Hb F, G gamma/ A gamma, i antigen, carbonic anhydrase, MCV, other enzymes). Erythroid progenitors will be cultured in vitro, in plasma clots, methyl cellulose, and flasks of adherent and suspension layers. Hb F and G gamma/A gamma synthesis will be measured in progenitor-derived colonies. The influence of variables such as time in culture; source, amount, and time of addition of erythropoietin; source and timing of T-cell factors found in various conditioned media; and cellular interactions such as adherent cells and monocytes, will all be determined with regard to hemoglobin synthesis. Material will be studied during normal ontogeny, as well as from patients with hemoglobinopathies, and with various levels of bone marrow failure syndromes.